Fiber optic devices having volume bragg grating elements

ABSTRACT

Fiber optic devices including volume Bragg grating (VBG) elements are disclosed. A fiber optic device may include one or more optical inputs, one or more VBG elements, and one or more optical receivers. Methods for manufacturing VBG elements and for controlling filter response are also disclosed. A VBG chip, and fiber optic devices using such a chip, are also provided. A VBG chip includes a monolithic glass structure onto which a plurality of VBGs have been recorded.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit under 35 U.S.C. § 119(e) of U.S.Provisional patent application No. 60/365,032, filed Mar. 15, 2002, thedisclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention is related generally to fiber optic devices. Inpreferred embodiments, the invention provides fiber optic devices havingone or more volume Bragg grating (VBG) elements, and methods for makingsuch VBG elements.

BACKGROUND OF THE INVENTION

[0003] Light wavelength selectivity of thick periodic structures was,historically, studied first in x-ray diffraction on crystalline solids.It was recognized that such selectivity arises due to the coherentaddition of the light energy diffracted by individual layers formingprecisely spaced stacks, such as that of the atomic layers of acrystalline lattice. The name of phenomenon, “Bragg diffraction,” wasgiven in recognition of the studies of it performed by Bragg.

[0004] Later, largely the same behavior was observed during thediffraction of light at optical wavelengths on the acoustic waves of theappropriate frequencies created inside optically transparent solidmedia. Acoustic waves create a periodic modulation of the index ofrefraction of a dielectric material via perturbation of its density. Asa result, an acoustic wave can be used to manipulate light based on itswavelength. Thus, it functions as a wavelength filter.

[0005] Acoustic perturbation, however, is of a temporal nature, andrelaxes completely after its source is extinguished and with itdisappears the filter. Long-lasting Bragg gratings were first utilized,perhaps, with the invention of full-color holography. It employedrelatively thick films of dichromatic gelatins (DCG) for holographicrecording of color-realistic images of 3-D objects by using lasers ofdifferent colors. Subsequent reconstruction of images with conventionalwhite light sources became possible due to the wavelength selectivityproperty of volume Bragg gratings. However, to the inventors' knowledge,even though the wavelength selectivity of the volume Bragg gratings wasthe underlying mechanism that enabled white-light reconstruction ofthick-layer DCG display holograms, their utility for separating,combining, or otherwise manipulating specific wavelengths of light withthe intention of achieving practical device functionality has not beenrecognized.

[0006] Use of volume Bragg gratings (VBG) recorded in doped lithiumniobate photorefractive crystals for filtering light at opticalwavelengths was adopted in construction of solar and lidar filters usedto isolate light at a particular wavelength from the broad bandbackground. A principal issue, however, is that recording of suchfilters must be performed at the same wavelength at which the filterwill subsequently operate. As a result, the use of these filters islimited to a very limited range of wavelengths where sufficientlypowerful lasers exist. Furthermore, the list of appropriate recordingmaterials is confined to two or three narrow classes of photorefractivematerials, which often have physical properties that are unsuitable fortheir intended mode of operation. For example, no material is known tothe inventors that would allow construction of practical functionalfiber-optic devices that would utilize volume Bragg grating filtersrecorded at wavelengths in the range of about 800-1650 nm.

[0007] This drawback can be partially overcome in photorefractivelithium niobate crystals when a VBG filter is recorded through adifferent surface than that used for its operation. By using thisapproach, filters can be constructed in lithium niobate that can operateat wavelengths that are useful for practical photonic devices, such as,for example, fiber-optic devices. Nonetheless, this approach is stillrather limited due to a number of factors. First, the usable wavelengthrange is limited to λ_(op)>n*λ_(rec) on the one side, and the nearinfrared absorption edge of the lithium niobate on the other. Also, forpractical devices, the bandwidth of the filter Δλ is limited by themaximum refractive index modulation achievable in that material (or itsdynamic range, Δn): Δλ<(λ_(op))*Δn/2n. This factor substantially limitsthe usefulness of this type of filter. This approach also requires theuse of at least two (and typically four) polished surfaces that areorthogonal to each other, which increases the complexity of the filtermanufacturing process and its cost. Additionally, the wavelength of thefilter is substantially fixed to the value determined by the anglebetween the recording beams in the holographic setup. As a result, thewavelength must be controlled precisely for any practical device and is,therefore, unique for a particular wavelength or information-carrying“channel” of light, which complicates the issues in manufacturing ofthese elements.

SUMMARY OF THE INVENTION

[0008] An embodiment of the invention includes a fiber optic devicecomprising an optical input that provides input radiation havingwavelength in a fiber optic range. A transmissive volume Bragg grating(VBG) element redirects the input radiation to an optical receiver. Asecond optical receiver may be provided to receive radiation transmittedthrough the VBG element, the transmitted radiation having a secondwavelength in the fiber optic range. The device may include a secondtransmissive VBG element that redirects radiation transmitted throughthe first VBG element, the transmitted radiation having a secondwavelength in the fiber optic range. A second optical receiver may beprovided to receive the redirected transmitted radiation. A thirdoptical receiver may be provided to receive radiation transmittedthrough the second VBG element, the second transmitted radiation havinga third wavelength in the fiber optic range. The device may include asecond optical input that provides second input radiation having anotherwavelength in the fiber optic range.

[0009] Another embodiment of the invention provides a fiber optic devicecomprising an optical input that provides input radiation having aplurality of wavelengths in a fiber optic range, and a volume Bragggrating (VBG) element made of sensitized silica glass. The VBG elementreceives the input radiation and redirects radiation having a firstwavelength to an optical receiver.

[0010] In another embodiment, a fiber optic device comprising an opticalinput and a plurality of VBG elements is provided. A first VBG elementreceives input radiation and redirects first redirected radiation havinga first wavelength in the fiber optic range. A second VBG elementreceives first transmitted radiation from the first VBG element andredirects second redirected radiation having a second wavelength in thefiber optic range. A first optical receiver receives the firstredirected radiation and a second optical receiver receives the secondredirected radiation. The VBG elements may be disposed along an opticalaxis of the fiber optic device. A face of the first VBG element may belaminated to a face of the second VBG element.

[0011] A fiber optic device according to the invention may include anoptical input, a VBG element, and a reflector that reflects transmittedradiation received from the VBG back into the VBG such that the VBGredirects second redirected radiation to a first optical receiver.

[0012] A method for controlling filter response is also provided. Such amethod includes providing a mask that corresponds to a desired filterresponse of a volume Bragg grating (VBG) element, and transmitting arecording beam through the mask. The recording beam is transmittedthrough a lens to a glass that is sensitive to a wavelength of therecording beam. The lens is adapted to perform an optical Fouriertransform of a transfer function associated with the mask. A secondrecording beam may be transmitted to the glass in combination with thefirst recording beam. The second recording beam may have generally thesame wavelength as the first recording beam, such that the first andsecond recording beams are coherent.

[0013] A method for manufacturing a VBG element by forming a large-waferVBG and segmenting the large-wafer VBG into a plurality of individualVBG elements is also provided. Each of the individual VBG elementsretains the index vector of the large-wafer VBG. The large-wafer VBGmay-be segmented by dicing the large-wafer VBG into the plurality ofindividual VBG elements.

[0014] Another method for manufacturing a VBG element includes forming afirst VBG element using a pair of recording beams and using a singlerecording beam to replicate the first VBG to form a second VBG.

[0015] A VBG chip, and fiber optic devices using such a chip, are alsoprovided. A VBG chip includes a monolithic glass structure onto which aplurality of VBGs have been recorded. The VBG chip may include a firstgrating recorded at a first location on the glass, wherein the firstgrating is adapted to receive incident light having a plurality ofwavelengths in a fiber optic range and to redirect first redirectedlight having a first wavelength in the fiber optic range. A secondgrating may be recorded at a second location on the glass to receive thefirst redirected light. The second grating may be adapted to redirectthe first redirected light out of the glass structure. Another gratingat another location on the glass may be adapted to receive transmittedlight from the first VBG, and to redirect light having a secondwavelength in the fiber optic range.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Certain preferred embodiments of the invention will now bedescribed in detail with reference to the figures. Those skilled in theart will appreciate that the description given herein with respect tothe figures is for exemplary purposes only and is not intended in anyway to limit the scope of the invention.

[0017]FIGS. 1A and 1B depict reflective and transmissive VBGs,respectively.

[0018]FIG. 2 demonstrates the transparency property of a VBG.

[0019]FIG. 3 is a schematic of a device according to the invention forcombining a plurality of optical inputs into a single optical fiberoutput.

[0020]FIG. 4 is a perspective view of a preferred embodiment of a deviceaccording to the invention.

[0021]FIG. 5 depicts the interior of a device such as shown in FIG. 4.

[0022]FIG. 6 is a perspective view of another preferred embodiment of adevice according to the invention.

[0023]FIG. 7 is a schematic diagram of a device such as shown in FIG. 6.

[0024]FIG. 8 is a perspective view of another preferred embodiment of adevice according to the invention.

[0025]FIG. 9 is a schematic diagram of a DWDM multi-source combineraccording to the invention.

[0026]FIG. 10 is a schematic diagram of an optical add-drop multiplexeraccording to the invention.

[0027]FIG. 11 is a schematic diagram of a multi-channel wavelengthmonitor according to the invention.

[0028]FIG. 12 depicts chain cascading of a plurality of VBGs in a fiberoptic device according to the invention.

[0029]FIG. 13 depicts lamination cascading of a plurality of VBGs in afiber optic device according to the invention.

[0030]FIG. 14 depicts a multiple path device according to the invention.

[0031]FIG. 15 depicts a method according to the invention forfabricating VBGs.

[0032] FIGS. 16A-C depict another method according to the invention forfabricating VBGs.

[0033]FIG. 17 depicts yet another method according to the invention forfabricating VBGs.

[0034]FIG. 18 depicts an integrated VBG “chip” according to theinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0035] Using sensitized silica glasses for manufacturing of VBG filters

[0036] One of the major problems in developing and using any kind ofpermanent VBG filters for practical applications has been theunavailability of a material or a class of materials possessing physicalproperties that are adequate for the practical applications. Forexample, the photorefractive electro-optic crystals, in which much ofthe research was conducted on the subject of VBGs, among other problems,are incapable of providing truly permanent, stable recording across awide temperature range. Furthermore, these crystals are stronglyanisotropic, which limits their usage substantially. For these reasons,an entire range of applications of VBG filters in general has not beensubstantially explored. In fact, to the inventors' knowledge, there isnot a single photonic device now in the market that uses VBG elements.

[0037] According to the invention, a previously unexplored class ofmaterials, the silicate photorefractive glasses (PRG), can be used toenable the design and manufacturing of practical devices based on VBGs,with special emphasis on photonic devices for fiber-optic applications.This type of materials substantially overcomes all of theabove-mentioned drawbacks of the previously studied materials andpossesses all the required properties to manufacture devices fordemanding applications exemplified by the fiber optics. These propertiesinclude, but are not limited to, the following: a) optical transparencyin the entire optical window from UV to mid-infrared; b) outstandinglongevity of the recorded gratings; c) outstanding thermal stability(>200 C); d) adequate dynamic range; e) excellent optical quality,including the achievable polishing quality of the elements made of thismaterial; d) low manufacturing costs; e) ability to be formed andprocessed in the adequate shapes and sizes (e.g., flat disks or wafers);f) refractive index isotropy. Compositions and processes formanufacturing such PRGs are described in U.S. Pat. No. 4,057,408, thedisclosure of which is hereby incorporated herein by reference in itsentirety.

[0038] Manufacturing of VBG Elements in Silica Glasses by RecordingHolographically at a Specific Wavelength and Using Them in Fiber-OpticDevices at an Arbitrary Wavelength

[0039] As described in the literature on the theory of Bragg diffractionin thick holograms (see, e.g., Kogelnik, H., “Coupled wave theory forthick hologram gratings,” The Bell System Technical Journal, November1969, 48(9), 2909-2947), there are two basic types of theVBGs—transmission and reflection, which are different in their mode ofoperation (see Kogelnik FIG. 4).

[0040]FIG. 1A depicts a reflective VBG 102 having a grating wave vector,A, in the horizontal direction as shown. An input light beam 104composed of light of a plurality of wavelengths λ₁, . . . λ_(N) isdirected toward the VBG 102 at a first angle α to the input face 102A ofthe VBG 102. The VBG 102 is formed such that it is transparent to allbut one of the wavelengths λ₁, . . . λ_(N). That is, the light beampropagates through the grating relatively unaffected, except that thelight having a certain wavelength, λ₁, is filtered out of the beam. As aresult, only that light 106 having wavelengths λ₂, . . . λ_(N) continuesthrough the VBG 102 and exits the VBG. 102 at a second angle β to theoutput face 102B of the VBG 102. Preferably, the VBG 102 is fabricatedso that the angle β at which the beam exits the VBG 102 is as near aspossible to the angle α at which it entered the VBG 102 (i.e., the beamcontinues along in a generally straight line). Light 108 havingwavelength λ₁, however, is reflected back at an angle γ from the inputface 102A of the VBG 102 because of the holography within the VBG 102.That is, the VBG 102 is fabricated such that the index of refractionvaries within the VBG 102 to allow light having wavelengths λ₂, . . .λ_(N) to continue through the VBG 102, and light having wavelength λ₁ tobe reflected back at a known angle. Methods for fabricating such VBGsare discussed in detail below.

[0041]FIG. 1B depicts a transmissive VBG 112 having a grating wavevector, A, in the vertical direction as shown. An input light beam 114composed of light of a plurality of wavelengths λ₁, . . . λ_(N) isdirected toward the VBG 112 at a first angle α to the input face 112A ofthe VBG 112. The VBG 112 is formed such that it is transparent to allbut one of the wavelengths. That is, the light beam propagates throughthe grating relatively unaffected, except that the light having acertain wavelength, λ₁, is filtered out of the beam. As a result, onlythat light 116 having wavelengths λ₂, . . . λ_(N) continues through theVBG 112 and exits the VBG 112 at a second angle β to the output face112B of the VBG 112. Preferably, the VBG 112 is fabricated so that theangle β at which the beam 116 exits the VBG 112 is as near as possibleto the angle α at which it entered the VBG 112 (i.e., it continues alongin a generally straight line). Light 118 having wavelength λ₁, however,exits the VBG 112 at a third angle γ to the output face 112B because ofthe holography within the VBG 112. That is, the VBG 112 is fabricatedsuch that the index of refraction varies within the VBG 112 to allowlight having wavelengths λ₂, . . . λ_(N) to continue relatively straightthrough the VBG 112, and light having wavelength λ₁ to be deflected asit passes through the VBG 112 such that it exits the VBG 112 at a knownangle β to the output face.

[0042] Wavelength filtering properties of transmission and reflectionVBGs are different primarily in the width of the filter that can beconstructed in an element of practical size. Generally, reflection thickvolume holograms have very narrow wavelength bandwidth, with the upperlimit determined by the dynamic range of the material, as describedabove in connection with the example of lithium niobate VBG filters.Conversely, transmission thick volume holograms generally have widerbandwidth, which, historically, has precluded their use for thegeneration of white light color display holograms.

[0043] Nonetheless, when recorded in a sufficiently thick slab of atransparent material (e.g., >1 mm), a method can be devised to recordtransmission VBGs that can achieve bandwidths sufficiently narrow forpractical photonic devices (e.g., bandwidth of 30 nm or less).

[0044] Another principal difference between reflection VBGs andtransmission VBGs is that the transmission type allows tuning of thecentral wavelength of the filter by adjusting the incident angle oflight upon the VBG. For that reason, a VBG filter can be recorded at onewavelength (e.g., in the UV range where silicate PRGs are sensitive) andoperate at another (e.g., in the 850 nm to 1650 nm range typicallyemployed in various fiber-optic devices). This can be achieved withoutthe limitations of recording through an orthogonal side of the element,described above for the case of the lithium niobate VBG filters. Thismeans that: a) the range of the usable wavelengths is practicallyunlimited; b) wider bandwidths are readily available; c) there is noneed for polishing additional surfaces.

[0045] The use of permanent transmission VBGs as band-pass filters formanipulation of wavelengths in photonic devices, exemplified by thefiber-optic active and passive components, has not been explored so farprobably for one or more of the following reasons: a) strong anisotropyof the material (e.g., inorganic electro-optic photorefractivecrystals); b) impossible to manufacture in sufficiently thick layers (>1mm, e.g., DCG); c) impossible to achieve sufficient optical quality ofthe bulk material and/or polishing quality of the surfaces (e.g.,photo-polymers); d) insufficient temperature stability.

[0046]FIG. 2 demonstrates the transparency property of a VBG 200 inwhich an input light beam 202 composed of light of a plurality ofwavelengths λ₁, . . . λ_(N) is directed toward the VBG 200, through alens 204, along an optical axis, x, of the device. As shown, the inputlight beam 202 can be emitted from an optical fiber 210. The VBG element200 is fabricated such that the index of refraction varies within theVBG 200 to allow light 206 having wavelengths λ₃, . . . λ_(N) tocontinue relatively straight through the VBG 200, through a lens 208,and into a receiver 212, which can be another output optical fiber, forexample, as shown. Light 214 having wavelength λ₁, however, is reflectedback at a first angle α from the input face 200A of the VBG 200.Similarly, light 216 having wavelength λ₂ is reflected back at a secondangle β from the input face 200A of the VBG 200 because of theholography within the VBG 200.

[0047] FIGS. 3-5 depict a preferred embodiment of a fiber optic device300 according to the invention for combining a plurality of opticalfiber inputs 311-314 into a single optical fiber output 310. As shown,the device 300 includes four optical inputs 311-314, which can beoptical fibers, for example. Each optical input 311-314 carries light301-304 of a different wavelength λ₁-λ₄. The device 300 also includesthree VBG elements 330, 332, 334. Light 301 from the first input 311,having wavelength λ₁, is transmitted into the interior of the device300, where it is deflected via a first deflector 320 (such as a mirror,for example) such that it enters the first VBG element 330 at a knownangle. As shown, the light travels along the optical axis of the device,and enters the VBG 330 at an angle of 90° to the input face of the VBG330. The first VBG 330 is transparent to light having wavelength ), sothe light having wavelength λ₁ exits the first VBG 330 at an angle of90° with the output face of the VBG 330.

[0048] Light 302 from the second input 312, having wavelength λ₂, istransmitted into the interior of the device 300, where it is deflectedvia a second deflector 322 such that it enters the first VBG element 330at a known angle. The first VBG 330 deflects the light having wavelengthλ₂ such that the light having wavelength λ₂ exits the first VBG 330 atan angle of 90° with the output face of the VBG 330 and, therefore, iscombined with the light having wavelength λ₁.

[0049] Similarly, light 303 from the third input 313, having wavelengthλ₃, is transmitted into the interior of the device 300, where it isdeflected via a third deflector 324 such that it enters the second VBGelement 332 at a known angle. The second VBG 332 deflects the lighthaving wavelength λ₃ such that the light having wavelength λ₃ exits thesecond VBG 332 at an angle of 90° with the output face of the second VBG332. The second VBG 332 is transparent to light having wavelength λ₁ orλ₂. Consequently, the light having wavelength λ₃ is combined with thelight having wavelength λ₁ and λ₂.

[0050] Similarly, light 304 from the fourth input 314, having wavelengthλ₄, is transmitted into the interior of the device 300, where it isdeflected via a fourth deflector 326 such that it enters the third VBGelement 334 at a known angle. The third VBG 334 deflects the lighthaving wavelength λ₄ such that the light having wavelength λ₄ exits thethird VBG 334 at an angle of 90° with the output face of the third VBG334. The third VBG 334 is transparent to light having wavelength λ₁, λ₂,or λ₃. Consequently, the light having wavelength λ₄ is combined with thelight having wavelength λ₁, λ₂, and λ₃.

[0051] Thus, an output light beam 306 composed of light have wavelengthsλ₁, λ₂, λ₃, and λ₄ can be formed using a plurality of VBG elements. Theoutput light beam 306 is received by an optical receiver 310, such as anoptical fiber. It should be understood that, by reversing the directionof the light flow, a device as shown in FIG. 5 can be used to generate aplurality of output light beams, each having a known wavelength, from aninput light beam composed of light having a plurality wavelengths.

[0052]FIGS. 6 and 7 depict a preferred embodiment of a triplexerbi-directional transmitter/receiver 600 according to the invention. Asshown, the device 600 includes an optical input 613, two optical outputs611, 612, and a bi-directional optical carrier 614, each of which can bean optical fiber, for example. The bi-directional carrier 614 carrieslight 604 having wavelengths λ₁ . . . λ₃ as shown. The first output 611carries light 601 of wavelength λ₁ to a first receiver 621. The secondoutput 612 carries light 602 of wavelength λ₂ to a second receiver 622.The optical input 613 carries light 603 of wavelength λ₃ from a source623.

[0053] The VBG 610 is fabricated such that it is transparent to lighthaving wavelength λ₃, which is transmitted to the VBG 610 via theoptical input 613. The VBG 610 can also be fabricated such that itdeflects light 601 having wavelength λ₁ and light 602 having wavelengthλ₂. The light 601 having wavelength λ₁ can be received by a firstoptical receiver 621, and the light having wavelength λ₂ can be receivedby a second optical receiver 622. The bi-directional carrier 614 carrieslight 604 having wavelength λ₁ and wavelength λ₂ in a first direction(toward the VBG) and wavelength λ₃ in a second direction (away from theVBG).

[0054]FIG. 8 depicts a preferred embodiment of a Xenpak form-factor CWDMtransmitter 800 according to the invention. As shown, the device 800includes an optical input 811 and four optical outputs 812-815, each ofwhich can be an optical fiber, for example. The optical input carrieslight having wavelengths λ₁ . . . λ₄. The first output 811 carries lightof wavelength λ₁; the second output 812 carries light of wavelength λ₂;the third output 813 carries light of wavelength λ₃; and the fourthoutput 814 carries light of wavelength λ₄.

[0055]FIG. 9 is a schematic diagram of a DWDM multi-source combiner 900according to the invention. As shown, the multi-source combiner 900includes four optical inputs 911-914, such as optical fibers, forexample. Each optical input 911-914 carries light of a differentwavelength λ₁, . . . λ₄ as shown. The device 900 also includes two VBGelements 932, 934. Light 901 from the first input 911, having wavelengthλ₁, is transmitted, preferably through a lens 921, such that it entersthe first VBG element 932 at a first known angle α. The first VBG 932 isfabricated such that the light 901 having wavelength λ₁ is deflectedfrom the first VBG 932 along the optical axis x of the device 900.Similarly, light 902 from the second input 912, having wavelength λ₂, istransmitted, preferably through a lens 922, such that it enters thefirst VBG element 932 at a second known angle β. The first VBG 932 isfabricated such that the light 902 having wavelength λ₂ is alsodeflected from the first VBG 932 along the optical axis x of the device900.

[0056] Light 903 from the third input 913, having wavelength λ₃, istransmitted, preferably through a lens 923, such that it enters thesecond VBG element 934 at a third known angle γ. The second VBG 934 isfabricated such that the light 903 having wavelength λ₃ is deflectedfrom the VBG 934 along the optical axis x of the device 900. Similarly,light 904 from the fourth input 914, having wavelength λ₄, istransmitted, preferably through a lens 924, such that it enters thesecond VBG element 934 at a fourth known angle δ. The second VBG 924 isfabricated such that the light 904 having wavelength λ₄ is alsodeflected from the second VBG 924 along the optical axis x of the device900. The first VBG 932 is transparent to light having wavelength λ₃ andλ₄. Thus, light beams having respective wavelengths λ₁, λ₂, λ₃, and λ₄can be combined into a single optical beam 905, which can then betransmitted, preferably through a lens 925, to an optical receiver 915,such as an optical fiber, for example.

[0057]FIG. 10 is a schematic diagram of a free-space optical add-dropmultiplexer (OADM) 1000 according to the invention. As shown, the OADM1000 includes an optical input 1011 , such as an optical fiber, forexample, that carries light 1001 having wavelengths λ₁, . . . λ_(N) asshown. As shown in FIG. 10, the OADM 1000 includes a VBG element 1020that is fabricated to reflect light 1002 having wavelength λ₁. The VBG1020 is transparent to light having wavelengths λ₂, . . . λ_(N). Thelight beam 1001 from the first input 1011 is incident onto the VBG 1020at a first angle α to a first face 1020A of the VBG element 1020.Consequently, a light beam 1002 having wavelength λ₁ is deflected at asecond angle β from the face 1020A of the VBG element 1020. The lightbeam 1002 having wavelength λ₁ is thus “dropped” from the input signal,and can be directed to an optical receiver 1012, such as another opticalfiber, for example.

[0058] The OADM 1000 also includes an additional input 1013, which canbe an optical fiber, for example, that carries a light beam 1003 havingwavelength λ_(N+1). The light beam 1003 having wavelength λ_(N+1) isincident onto the VBG 1020 at an angle γ to a second face 1020B of theVBG 1020. The VBG element 1020 is fabricated to reflect light havingwavelength λ_(N+1) from the second face 1020B such that the light 1003from the additional input 1013 is combined with the light from the firstinput to form an output light beam 1004 having wavelengths λ₂, . . .λ_(N+1). The output light beam 1004 can be directed to an opticalreceiver 1014, such as another optical fiber, for example.

[0059]FIG. 11 is a schematic diagram of a multi-channel wavelengthmonitor 1100 according to the invention. As shown, the multi-channelwavelength monitor 1100 includes an optical input 1115 that carrieslight 1005 having wavelengths λ₁, . . . λ_(N). The monitor 1100 alsoincludes two VBG elements 1132, 1134. The input light beam 1105 istransmitted, preferably through a lens 1125, such that it enters thefirst VBG element 1132 at a first known angle (preferably, along theoptical axis x of the device 1100, that is, 90° to the face 1132A of thefirst VBG 1132). The first VBG 1132 is fabricated such that the light1101 having wavelength λ₁+Δ is deflected from the first VBG 1132 at afirst angle α, and light 1102 having wavelength λ₁−Δ is deflected fromthe first VBG 1132 at a second angle β. The first VBG 1132 istransparent to the rest of the wavelengths in the input beam 1105. Light1101 having wavelength λ₁+Δ may be received by an optical receiver 1111,and light 1102 may be received by an optical receiver 1112.

[0060] The light beam is then transmitted to the second VBG element1134, which is fabricated such that the light 1103 having wavelengthλ₂+Δ is deflected from the second VBG 1134 at a first angle γ, and light1104 having wavelength λ₂−Δ is deflected from the second VBG 1134 at asecond angle δ. Light 1103 having wavelength λ₂+Δ may be received by anoptical receiver 1113, and light 1104 having wavelength λ₂−Δ may bereceived by an optical receiver 1114. The second VBG 1134 is transparentto the rest of the wavelengths in the beam. The output beam 1106 canthen be received, preferably through a lens 1126, by an optical receiver1116, which can be another optical fiber, for example.

[0061] Methods for Packaging Devices With Large Channel Counts Using VBGFilters

[0062] One of the main advantages of VBG filters and, indeed, theirunique property is the ability to record multiple filters sharing thesame volume of the material. This allows for the fabrication of devicesof very small size and unique functionality. Nevertheless, the number ofgratings that can share the same volume, known as the multiplexingnumber, or the M/#, in the holographic memory field, is limited by thedynamic range of the material. For that reason, for practical materialssuitable for manufacturing of VBG filters, that number will typically berather limited (a realistic estimate is around 4 filters for a 4 mmthick element). Furthermore, fabrication of VBG filters with a largernumber of gratings becomes progressively more complex, while at the sametime reducing the flexibility in packaging them in a device. Inaddition, when sharing the same volume, the combined effect of the VBGscan be obtained via the coherent addition of the effects of theindividual gratings, which results in the appearance of cross-terms,leading sometimes to undesirable side effects. It is, therefore,desirable to have a practical method for manufacturing devices withsufficiently large channel count. According to one aspect of theinvention, fiber optic devices can be fabricated which can have abasically unlimited number of channels while using very simple VBGelements as building blocks.

[0063]FIG. 12 depicts a device according to the invention in which anynumber of transmissive VBG filters can be combined to construct a devicewith an arbitrary channel count and for an arbitrary set of wavelengths.Preferably, the VBG filters are identical, thereby reducing the cost offabrication. In this approach, the individual VBG elements arepositioned on the main optical axis of the device and their tilt anglesare adjusted individually in order to tune it to the peak wavelength ofthe desired channel. This approach may be referred to as “chaincascading.”

[0064] As shown in FIG. 12, an optical input 1214 carries light 1204having wavelengths λ₁, . . . λ_(N). Light 1204 is incident on a firstVBG element 1232 along the optical axis x of the device. The first VBG1232 is fabricated such that the light 1201 having wavelength λ₁ isdeflected from the first VBG 1232 at a first angle α to the exit face1232B of the VBG 1232. As shown, the VBG 1232 is positioned such thatits grating vector A and exit face 1232B are perpendicular to theoptical axis x of the device. Thus, the light 1201 having wavelength λ₁is deflected from the first VBG 1232 at an angle 90-α to the opticalaxis x of the device. The device may include a first optical receiver1211 that receives the deflected beam 1201. The first VBG 1232 istransparent to the rest of the wavelengths λ₂, . . . λ_(N) in the inputbeam 1204, such that a transmitted beam 1205 having wavelengths λ₂, . .. λ_(N) is transmitted through the VBG 1232 along the optical axis x.

[0065] The transmitted beam 1205 is incident on a second VBG element1234. The second VBG 1234 may be fabricated, like the first VBG 1234,such that light having wavelength λ₁ would be deflected from the secondVBG 1234 at a first angle α to the exit face 1234B of the VBG 1234. Asshown, the VBG 1234 is positioned such that its grating vector A andexit face 1234B are at a known angle θ (>90°) to the optical axis x ofthe device. Light 1202 having wavelength λ₂ is deflected from the secondVBG 1234 at a known angle β to the exit face 1234B of the VBG 1234 (and,therefore, at a known angle to the optical axis x). The device mayinclude a second optical receiver 1212 that receives the deflected beam1202. The second VBG 1234 is transparent to the rest of the wavelengthsλ₃, . . . λ_(N) in the transmitted beam 1205, such that a secondtransmitted beam 1206 having wavelengths λ₃, . . . λ_(N) is transmittedthrough the VBG 1234 along the optical axis x.

[0066] The transmitted beam 1206 is incident on a third VBG element1236. The third VBG 1236 may be fabricated, like the first VBG 1232,such that light having wavelength λ₁ would be deflected from the thirdVBG 1236 at a first angle α to the exit face 1236B of the VBG 1236. Asshown, the VBG 1236 is positioned such that its grating vector A andexit face 1236B are at a known angle φ (<90°) to the optical axis x ofthe device. Light 1203 having wavelength λ₃ is deflected from the thirdVBG 1236 at a known angle γ to the exit face 1236B of the VBG 1236 (and,therefore, at a known angle to the optical axis x). The device mayinclude a third optical receiver 1213 that receives the deflected beam1203. The third VBG 1236 is transparent to the rest of the wavelengthsλ₄, . . . λ_(N) in the transmitted beam 1206, such that a thirdtransmitted beam 1207 having wavelengths λ₄, . . . λ_(N) is transmittedthrough the VBG 1236 along the optical axis x. The device may include afourth optical receiver 1215 that receives the transmitted beam 1207.

[0067]FIG. 13 depicts a device according to the invention that includesa complex VBG filter element that has been fabricated from a number ofsimple, possibly identical, VBG elements. In function it is similar tothe device described above in connection with FIG. 12 but instead ofbeing positioned and adjusted individually in the package, the elementscan be properly positioned in a suitable fixture in direct physicalcontact with one another and then permanently bonded together, usingsuitable bonding materials that are well known in the art, thus creatinga single compounded element with complex functionality.

[0068] The positioning of the individual VBG elements with respect toone another in such an arrangement can be important to the usefulness ofthe assembly. Methods of exercising such control can include: a) propersurface preparation of the wafers of the recording material, such aspolishing, parallelism of the surfaces etc.; b) proper rotationalorientation of the elements with respect to each other during thebonding procedure; c) use of calibrated spacers to the adjust relativeangle between the individual VBG elements; d) precise control of thetilt angle of the wafer with respect to the recording laser beams duringthe holographic recording process.

[0069] This approach, referred to as “lamination cascading,” enables theachievement of the same density of the grating packing in the samepackage volume as the direct multiplexing of the filters during therecording process, but without the need of multiple exposures andwithout the physical overlap, and thus interference, of the individualfilters in the bulk of the material.

[0070] As shown in FIG. 13, an optical input 1314 carries light 1304having wavelengths λ₁, . . . λ_(N). Light 1304 is incident on a seriesof VBG elements 1332-1334 along the optical axis x of the device. TheVBG elements 1332-1334 are fabricated such that light 1301 havingwavelength λ₁ is deflected from the third VBG 1334 at a first angle α tothe exit face 1334B of the VBG 1334, light 1302 having wavelength λ₂ isdeflected from the third VBG 1334 at a second angle β to the exit face1334B of the VBG 1334, and light 1303 having wavelength λ₃ is deflectedfrom the third VBG 1334 at a third angle γ to the exit face 1334B of theVBG 1334. The device may include a first optical receiver 1311 thatreceives the deflected beam 1301, a second optical receiver 1312 thatreceives the deflected beam 1302, and a third optical receiver 1313 thatreceives the deflected beam 1303. The VBGs 1332-1334 may be transparentto the rest of the wavelengths λ₄, . . . λ_(N) in the input beam 1304,such that a transmitted beam 1305 having wavelengths λ₄, . . . λ_(N) istransmitted through the VBGs 1332-1334 along the optical axis x. Thedevice may include a fourth optical receiver 1314 that receives thetransmitted beam 1304.

[0071]FIG. 14 depicts a device according to the invention in which anysimple, individual VBG element can be used for processing severalwavelength channels by allowing multiple paths through it in differentdirections. In its simplest form, the so-called “double passconfiguration” functions as follows:

[0072] A series of simple individual VBG elements is positioned in lineas described above in connection with FIG. 12. A mirror is placed at theend of the chain of the elements, which reflects the transmitted lightback onto the same elements. This has the effect of folding the chain ofthe elements back onto itself. The mirror angle is adjusted slightly, sothat the angle of the back-reflected light is somewhat different thanthe forward-propagating light. This angle is adjusted in such a way asto tune the center wavelength of the VBG filters to the desired value.

[0073] In such an implementation, the method allows using each of theVBG elements more than once, thus effectively increasing the number offilters without increasing the number of VBG elements, and therebyenabling the overall size of the package to remain practically the same.Multiple path folding is also possible if an additional mirror is usedin the beginning of the chain of the VBG elements, slightly offset fromthe axis in angle and space.

[0074] As shown in FIG. 14, an optical input 1415 carries light 1405having wavelengths λ₁, . . . λ_(N) Light 1405 is incident on a first VBGelement 1432 along the optical axis x of the device. The first VBG 1432is fabricated such that light 1401 having wavelength λ₁ is deflectedfrom the first VBG 1432 at an angle α to the exit face 1432B of thefirst VBG 1432. The first VBG 1432 is transparent to the rest of thewavelengths λ₂, . . . λ_(N) in the input beam 1405, such that atransmitted beam 1406 having wavelengths λ₂, . . . λ_(N) is transmittedthrough the VBG 1432 along the optical axis x.

[0075] The transmitted beam 1406 is incident on a second VBG element1434. The second VBG 1434 is fabricated such that light 1402 havingwavelength λ₂ is deflected from the second VBG 1434 at an angle β to theexit face 1434B of the second VBG 1434. The second VBG 1434 istransparent to the rest of the wavelengths λ₃, . . . λ_(N i)n thetransmitted beam 1406, such that a transmitted beam 1407 havingwavelengths λ₃, . . . λ_(N) is transmitted through the VBG 1434 alongthe optical axis x.

[0076] The transmitted beam 1407 is directed toward a mirror 1420, whichis disposed at an angle φ to the optical axis x of the device. Thereflected beam 1408 is incident on the second VBG 1434 at an angle φ tothe exit face 1434B. The second VBG 1434 is fabricated such that light1403 having wavelength λ₃ is deflected from the second VBG 1434 at anangle γ to the entrance face 1434A of the second VBG 1434. The secondVBG 1434 is transparent to the rest of the wavelengths λ₄, . . . λ_(N)in the reflected beam 1408, such that a reflected beam 1409 havingwavelengths λ₄, . . . λ_(N) is transmitted through the VBG 1434.

[0077] The reflected beam 1409 is incident on the first VBG 1432 at anangle φ to the exit face 1432B. The first VBG 1434 is fabricated suchthat light 1404 having wavelength λ₄ is deflected from the first VBG1432 at an angle δ to the entrance face 1432A of the first VBG 1432. Thefirst VBG 1432 is transparent to the rest of the wavelengths λ₅, . . .λ_(N) in the reflected beam 1409, such that a reflected beam 1410 havingwavelengths λ₅, . . . λ_(N) is transmitted through the VBG 1432.

[0078] The device may include a first optical receiver that receives thedeflected beam 1401, a second optical receiver that receives thedeflected beam 1402, and a third optical receiver that receives thedeflected beam 1403, and a fourth optical receiver that receives thedeflected beam 1404. The device may also include a fifth opticalreceiver that receives the reflected beam 1410.

[0079] It should be understood that any of the techniques describedabove can be optimized to take maximum advantage of VBG properties suchas: transparency to all but one wavelength, angular tunability,functionality distributed over the volume of a thick material, materialrigidity and dimensional stability, excellent polishing qualities, andthe like.

[0080] Methods for Economically Manufacturing VBG Elements

[0081] In the manufacturing of VBG elements, it is typically desirableto minimize the costs of production of such elements. For that reason,holographic recording of each element individually is likely to becost-prohibitive for most or all of the high-volume applications. Anumber of methods according to the invention for cost-effectiveproduction of such elements will now be described.

[0082] A first such method, depicted in FIG. 15, exploits the uniqueproperty of a hologram, whereupon each fractional piece of the recordedhologram possesses full and complete information about the recordedobject. When applied to the VBG filters recorded on the PRG plates, itmeans that each piece of such plate, or wafer, should have the samefiltering properties as the wafer in whole. For that reason, alarge-size wafer 1500 can be diced, using a suitable cutting device,such as a saw, for example, into a large number of relatively smallindividual VBG elements 1502, each with complete filter functionality.In following this process, one could significantly reduce the number ofrecording and testing operations, thereby reducing the manufacturingcosts of the VBG elements.

[0083] A second cost-reduction method according to the invention appliesto the repetitive fabrication of the filter with identical properties.Such an approach is particularly suitable for high-volume productionenvironments. In such circumstances reproduction of a filter with acomplex shape, which may require, for example, multiple exposure stepsto achieve the complete control over its spectral shape, may result in aprohibitively long and complex manufacturing operations. However, sinceholography allows true and complete reconstruction of the recordedwavefront, it is, therefore, possible to record a hologram of thereconstructed wavefront, rather than the true original, to achieve thesame result.

[0084] This approach includes: a) placing a “virgin” recording waferdirectly behind a recorded “master” hologram; and b) directing thereference beam onto the master hologram in exactly the same fashion asduring the recording of the master. The transmitted reference wave andthe reconstructed object wave interfere again behind the masterhologram. Consequently, a new hologram is recorded on the virgin wafer,which is an exact replica of the master.

[0085] The advantages of this method include but are not limited to thefollowing: a) better stability (not sensitive to the phasefluctuations); b) simpler setup (no filter shape control required); c)no polishing on the virgin wafer is required, if it is placed in directcontact with the master and an index matching fluid is used on theinterface; and d) shorter cycle times (higher throughput).

[0086] Methods to Control the Filter Response Function

[0087] When used in practical applications such as in fiber-opticdevices, for example, the spectral shape of a filter can be used tomanipulate the wavelengths of light in a desired fashion. The filtershape can determine such device parameters as adjacent channelisolation, cross-talk, suppression ratio, etc. The ability to controlthe spectral shape of the VBG filters, therefore, can make thedifference between a practically usable device and a practically uselessone.

[0088] As follows from the general theory of Bragg diffraction (seeKogelnik), the spectral shape of the filter created by a VBG is relatedvia a Fourier transform to the amplitude and phase envelope of the VBGalong the general direction of propagation of the affected light wave.It is, therefore, desirable to be able to control both in order tocreate a filter with a desired spectral shape.

[0089] A method according to the invention for controlling the spectralshape of a VBG filter relates to the use of the Fourier transformproperty of a lens and the phase capturing ability of the holographicrecording method. As depicted in FIG. 16A, a method 1600 for creating aVBG filter with any desired spectral shape can be performed as follows.A mask 1602 representing the desired filter shape is placed in the frontfocal plane of a lens 1604 situated in the object arm of the holographicrecording setup. The recording media sample 1606 (e.g., a glass wafer)is placed in the back focal plane of the same lens. The plane-wavefrontreference beam of the holographic recording setup overlaps with theobject beam on the sample, subtending it at an angle required by thetarget operational wavelength of the VBG filter being recorded.

[0090] When positioned as described, the lens creates a true Fouriertransform of the mask directly on the recording medium. Via a coherentinterference with the plane reference wave, both the amplitude and thephase of the Fourier transform are transferred to the amplitude andphase envelope of the VBG imprinted on the recording material. Whenreconstructed, or “read,” with a light beam nearly normal to therecorded grating planes, the spectral response of the VBG filter thusrecorded will take the shape of the masks placed in the front focalplane of the lens.

[0091] This method allows for a single exposure recording of a filterwith practically arbitrary complexity of the shape of the spectralresponse function and is referred to as the “parallel method” or the“holographic filter imprinting method.”

[0092] A holographic filter imprinting method according to the inventioncan be similarly applied to the task of shaping the filter responsefunction of transmission VBG filters. It may be accomplished by choosinga proper orientation of the apodizing mask relative to the direction ofthe grating planes and, similarly, by choosing the proper entrance andexit faces on the VBG element.

[0093] Exemplary methods asks for creating transmissive and reflectiveVBGs are depicted in FIGS. 16B and 16C respectively. As shown, a mask1612 having a slit 1614 can be placed in the front focal plane of thelens. Light shone through the mask will generate a square wave 1616. Asthe light passes though the lens, the Fourier transform 1618 of thesquare wave will be imprinted on the sample 1620. When reconstructed,the spectral response of the pattern 1622 recorded on the VBG filter1620 will take the shape of a square wave 1624. Depending on theorientation of the mask relative to the grating plane, the VBG can bemade reflective (as shown in FIG. 16B) or transmissive (as shown in FIG.16C).

[0094] Method for Controlling the Shape of Transmissive VBG Filters

[0095] Furthermore, when dealing with VBG filters functioning in thetransmission geometry, a different approach can be taken in order tomanipulate the spectral shape of the filter. In this case, the method,which is depicted in FIG. 17, comprises multiple, sequential exposuresof the same volume of the recording material. Each exposure wouldproduce a simple plane VBG, but after recording multiple gratings afilter of an arbitrary shape will be constructed via coherent additionof the recorded VBGs. Volume Bragg gratings are physical representationsof sinusoidal waves, and, therefore, their coherent sum is a Fouriertransform of an envelope function.

[0096] For that reason, a close representation of an arbitrary amplitudeand phase envelope function can be constructed via a series ofholographic exposures, provided appropriate control is exercised overboth the amplitude and the relative phase of the gratings recorded insuch series of exposures. Such control can be achieved via employingtechniques for active measurement and stabilization of the phase of therecorded VBGs.

[0097] As shown in FIG. 17, a virgin sample 1702 is subjected to a firstpair of incident beams 1712 and 1714. Beam 1712 is incident on theentrance face 1702A of the virgin sample 1702 at an angle α relative tothe entrance face 1702A (and, as shown, relative to the grating vectorA). Beam 1714 is incident on the entrance face 1702A of the virginsample 1702 at an angle β relative to the entrance face 1702A (and, asshown, relative to the grating vector A). Thus, a first holographicsample 1704 is formed having a first holographic image 1722.

[0098] The first holographic sample 1704 is then subjected to a secondpair of incident beams 1716 and 1718. Beam 1716 is incident on theentrance face 1704A of the first holographic sample 1704 at an angle γrelative to the entrance face 1704A (and, as shown, relative to thegrating vector A). Beam 1718 is incident on the entrance face 1704A ofthe first holographic sample 1704 at an angle δ relative to the entranceface 1704A (and, as shown, relative to the grating vector A). Thus, asecond holographic sample 1706 is formed having a second holographicimage 1724.

[0099] VBG Chip

[0100]FIG. 18 depicts an integrated VBG “chip” according to theinvention. As shown, the VBG chip 1800 is a monolithic glass structureinto which a plurality of holographic images or “gratings” have beenrecorded. An optical input 1815 carries light 1805 having wavelengthsλ₁, . . . λ_(N). Light 1805, which may be collimated, is incident on afirst grating 1822. Grating 1822 is recorded such that light 1801 havingwavelength λ₁ is deflected at an angle such that it is received bygrating 1832. Grating 1832 is recorded such that it deflects the light1801 out of the chip 1800 toward an optical receiver 1811. Grating 1822is transparent to the rest of the wavelengths λ₂, . . . λ_(N) in theinput beam 1805, such that a transmitted beam 1806 having wavelengthsλ₂, . . . λ_(N) is transmitted through the grating 1822.

[0101] The transmitted beam 1806 is incident on grating 1824, which isrecorded such that light 1802 having wavelength λ₂ is deflected at anangle such that it is received by grating 1834. Grating 1834 is recordedsuch that it deflects the light 1802 out of the chip 1800 toward anoptical receiver 1812. Grating 1824 is transparent to the rest of thewavelengths λ₃, . . . λ_(N) in the beam 1806, such that a transmittedbeam 1807 having wavelengths λ₃, . . . λ_(N) is transmitted through thegrating 1824.

[0102] Similarly, the transmitted beam 1807 is incident on grating 1826,which is recorded such that light 1803 having wavelength λ₃ is deflectedat an angle such that it is received by grating 1836. Grating 1836 isrecorded such that it deflects the light 1803 out of the chip 1800toward an optical receiver 1813. Grating 1826 is transparent to the restof the wavelengths λ₄, . . . λ_(N) in the beam 1807, such that atransmitted beam 1808 having wavelengths λ₄, . . . λ_(N) is transmittedthrough the grating 1826.

[0103] The transmitted beam 1808 is incident on grating 1828, which isrecorded such that light 1804 having wavelength λ₄ is deflected at anangle such that it is received by grating 1838. Grating 1838 is recordedsuch that it deflects the light 1804 out of the chip 1800 toward anoptical receiver 1814. Grating 1828 is transparent to the rest of thewavelengths λ₅, . . . λ_(N) in the beam 1808, such that a transmittedbeam 1809 having wavelengths λ₅, . . . λ_(N) is transmitted through thegrating 1828. The transmitted beam 1809 is directed toward an opticalreceiver 1815. As shown, each of the optical receivers 1811-1814 and1816 can be an optical fiber, for example. Any or all of the opticalreceivers 1811-1814 and 1816 can be bundled together to form an opticalfiber ribbon, for example.

[0104] A VBG chip as shown can be made according to the followingmethod. One or more incident beams are directed toward a first locationof a virgin sample (to form grating 1822, for example). Then, the beamsare turned off, and either the sample or the source of illumination ispositioned (e.g., the sample may be moved laterally and/or rotationallyas necessary) such that the incident beam(s) may now be directed towarda second location on the sample (to form grating 1824, for example).This process is repeated until all desired gratings have been recorded.

[0105] Thus, there have been described fiber optic devices comprisingvolume Bragg gratings and methods for fabricating the same. Thoseskilled in the art will appreciate that numerous changes andmodifications can be made to the preferred embodiments of the invention,and that such changes and modifications can be made without departingfrom the spirit of the invention. Examples of devices that can be madein accordance with the invention include, without limitation, 1×N lasersource combiners, multi-channel transmit/receive modules (includingtriplexers), optical add-drop multiplexers, terminal multiplexers,network monitors, wavelength lockers, tunable filters, tunable gainequalizers, dispersion compensators, and the like.

What is claimed:
 1. A fiber optic device, comprising: an optical inputthat provides input radiation having wavelength in a fiber optic range;a transmissive volume Bragg grating (VBG) element that redirects theinput radiation; and an optical receiver that receives the redirectedradiation.
 2. The fiber optic device of claim 1, further comprising: asecond optical receiver that receives transmitted radiation transmittedthrough the VBG element, the transmitted radiation having a secondwavelength in the fiber optic range.
 3. The fiber optic device of claim1, further comprising: a second transmissive VBG element that redirectstransmitted radiation transmitted through the VBG element, thetransmitted radiation having a second wavelength in the fiber opticrange; and a second optical receiver that receives the redirectedtransmitted radiation.
 4. The fiber optic device of claim 3, furthercomprising: a third optical receiver that receives second transmittedradiation transmitted through the second VBG element, the secondtransmitted radiation having a third wavelength in the fiber opticrange.
 5. The fiber optic device of claim 1, further comprising: asecond optical input that provides second input radiation having asecond wavelength in the fiber optic range, wherein the optical receiverreceives the second input radiation.
 6. The fiber optic device of claim5, wherein the VBG redirects the second input radiation toward theoptical receiver.
 7. The fiber optic device of claim 5, wherein thesecond optical input provides third input radiation having a thirdwavelength, and the VBG element redirects the third input radiation to asecond optical receiver.
 8. The fiber optic device of claim 1, whereinthe optical input comprises an optical fiber.
 9. A fiber optic device,comprising: an optical input that provides input radiation having aplurality of wavelengths in a fiber optic range; a volume Bragg grating(VBG) element made of sensitized silica glass that receives the inputradiation and redirects first redirected radiation having a firstwavelength in the fiber optic range; and an optical receiver thatreceives the first redirected radiation.
 10. A fiber optic devicecomprising: an optical input that provides input radiation having aplurality of wavelengths in a fiber optic range; a first volume Bragggrating (VBG) element that receives the input radiation and redirectsfirst redirected radiation having a first wavelength in the fiber opticrange; a second VBG element that receives first transmitted radiationfrom the first VBG element and redirects second redirected radiationhaving a second wavelength in the fiber optic range; a first opticalreceiver that receives the first redirected radiation; and a secondoptical receiver that receives the second redirected radiation.
 11. Thefiber optic device of claim 10, wherein the VBG elements are disposedalong an optical axis of the fiber optic device.
 12. The fiber opticdevice of claim 10, wherein a face of the first VBG element is laminatedto a face of the second VBG element.
 13. A fiber optic devicecomprising: an optical input that provides input radiation having aplurality of wavelengths in a fiber optic range; a volume Bragg grating(VBG) element that receives the input radiation and redirects firstredirected radiation having a first wavelength in the fiber optic range;a reflector that reflects first transmitted radiation received from theVBG back into the VBG such that the VBG redirects second redirectedradiation having a second wavelength in the fiber optic range; a firstoptical receiver that receives the first redirected radiation; and asecond optical receiver that receives the second redirected radiation.14. A method for controlling filter response, the method comprising:providing a mask that corresponds to a desired filter response of avolume Bragg grating (VBG) element; transmitting a first recording beamthrough the mask such that the first recording beam is transmittedthrough a lens to a glass that is sensitive to a wavelength of the firstrecording beam, wherein the lens is adapted to perform an opticalFourier transform of a transfer function associated with the mask. 15.The method of claim 14, further comprising: transmitting to the glass asecond recording beam in combination with the first recording beam,wherein the second recording beam has generally the same wavelength asthe first recording beam, such that the first and second recording beamsare coherent.
 16. A method for manufacturing a volume Bragg grating(VBG) element, the method comprising: forming a large-wafer VBG havingan index vector; and segmenting the large-wafer VBG into a plurality ofindividual VBG elements, wherein each of the individual VBG elementsretains the index vector of the large-wafer VBG.
 17. The method of claim16, wherein segmenting the large-wafer VBG comprises dicing thelarge-wafer VBG into the plurality of individual VBG elements.
 18. Amethod for manufacturing a volume Bragg grating (VBG) element, themethod comprising: forming a first VBG element using a pair of recordingbeams; and using a single recording beam to replicate the first VBG toform a second VBG.
 19. A volume Bragg grating (VBG) chip, comprising: amonolithic glass structure onto which a plurality of volume Bragggratings (VBGs) have been recorded.
 20. The VBG chip of claim 13,comprising a first grating recorded at a first location on the glass,wherein the first grating is adapted to receive incident light having aplurality of wavelengths in a fiber optic range and to redirect firstredirected light having a first wavelength in the fiber optic range. 21.The VBG chip of claim 14, comprising a second grating recorded at asecond location on the glass to receive the first redirected light,wherein the second grating is adapted to redirect the first redirectedlight out of the glass structure.
 22. The VBG chip of claim 14,comprising a second grating recorded at a second location on the glassto receive first transmitted light from the first VBG, wherein thesecond grating is adapted to redirect second redirected light having asecond wavelength in the fiber optic range.
 23. A fiber optic devicecomprising: an optical input that provides input radiation having aplurality of wavelengths in a fiber optic range; a monolithic glassstructure onto which a plurality of volume Bragg gratings (VBGs) havebeen recorded, at least one of the VBGs being adapted to redirect firstredirected light having a first wavelength in the fiber optic range; andan optical receiver that receives the redirected radiation.